Wireless communication systems may include a transceiver having at least one receiver and at least one transmitter connected to a common antenna through corresponding filters. The filters are often band pass filters, respectively included in the receive chain and the transmit chain of the transceiver, which filter out signal frequencies outside the corresponding receive passbands and transmit passbands. Signals with frequencies within a particular passband may be referred to as being in-band, while signals with frequencies outside the particular passband may be referred to as out-of-band (some of which may be in-band in other passbands). Modern wireless communication systems typically support multi-band, multi-standard and multi-mode receivers. Such systems also require low noise performance and high linearity for high data rates, and therefore include low noise amplifiers (LNAs). High linearity and low noise performance of an LNA is important, since the LNA is often the first component in a receiver chain, and therefore heavily influences performance of the other components in the receiver chain.
One useful type of LNA is a cascode LNA, which is a multiple stage amplifier including at least two transistors. The transistors may be any of various types of transistors, including field effect transistors (FETs) or bipolar junction transistors (BJTs), for example. For ease of explanation, the descriptions throughout will assume use of FETs, although it is understood that the BJTs or other types of compatible transistors may be substituted for the FETs without changing general operation of the circuit, as would be apparent to one of ordinary skill in the art. A cascode LNA may be implemented using a common source transistor connected at its drain to the source of a common gate transistor, for example.
Cascode LNAs with inductive source degeneration are widely used to improve noise performance for narrow-band applications. A conventional cascode LNA with inductive source degeneration includes a degeneration inductor connected between ground and the source of a first transistor (configured to receive an input voltage at its gate). However, in multi-band, multi-standard and/or multi-band receivers, out-of-band signals may be present, which may be interfering signals (e.g., “interferers” or “jammers”). The intermodulation and harmonics of such interfering signals can fall in-band of a particular passband, causing interference with the desired or wanted in-band signal.
Suppression of such in-band interference caused by the intermodulation and harmonics of interfering signals should be addressed in the LNA, since the interference may otherwise degrade receiver sensitivity and overall receiver performance further down the receiver chain. Due to the high linearity requirement, recent LNAs need to have additional linearization circuits, which consume more power and increase complexity, which are undesirable.
Accordingly, there is a need for providing an LNA capable of efficiently suppressing the second harmonic frequency, with little to no increase in power consumption and system complexity.